GEOMAGNETIC STUDY FOR TAR SAND EXPLORATION AT ILUBIRIN AND ITS ENVIRONS, SOUTHWESTERN, NIGERIA.

Abstract

A geomagnetic investigation has been carried out in order to provide structural information that can guide further tar sand exploration strategy in the southern part of Ondo state, southwestern Nigeria. Aeromagnetic data, Sheet 282 procured from the Nigerian Geological Survey Agency (NGSA) and ground magnetic profiling conducted along six traverses covering Oja-Baale, Boridele, Mulekangbo, Ilubirin, Ominla III and Egbe all of which falls within the Agbabu Tar Sand Belt were both utilized to explore for magnetic signatures that are indicative of structures and features significant for the emplacement of tar sand deposits or conduit for bitumen seepages in the area. Qualitative and quantitative interpretation of the aeromagnetic and ground magnetic profiles yielded useful comparable results that strengthen the information on suspected geologic structures and gross subsurface topography in the study area. The subtle anomalies of delineated structures in form of faults and dykes were enhanced by subjecting the magnetic data to filtering operations such as downward continuation, vertical gradient, horizontal gradients and tilt derivatives. Geo-magnetic sections generated from the straight slope analysis of the six profiles revealed that the depth to the magnetic basement varies between 120 m and 350 m within the basement complex and between 120 m and 380 m in the sedimentary basin. This analysis revealed sixteen (16) suspected fractures/faults in the study area; four (4) on traverse 1 at distances of 1000, 2200, 3200 and 5000 m, one (1) on traverse 2 at a distance of 1750 m, three (3) on traverse 3 at distance of 1250, 1550 and 1800 m, two (2) on traverse 4 at distances of 600 and 1750 m and another five (5) on traverse 6 at distances 800, 1500, 2800, 3450, 4800, and 5500 m along the profile. Three suspected dykes were equally delineated at distances 4500 and 6400 m on traverse 1 and at 2500 m along traverse 5. The depth estimates of these magnetic sources across the study area were obtained from three-dimensional (3D) Euler deconvolution of the aeromagnetic data to range from 50 to 350 m. Intrasedimentary features suspected from downward continuation filtered aeromagnetic data could be linked to the basement structures, which leads to the assumption that the faults within the sedimentary rock lies conformably over the basement and principally controlled by reactivation of the basement faults. Correlation of the available borehole data with these magnetic results shows increased saturation of the tar sand along the orientation of the suspected fault (F - F’) on the structural map which further indicates that the tar sand deposits in the area might be structurally controlled. Further exploration in the study area has been strongly recommended to take cognizance of this structural disposition in mapping the tar sand deposits.